The Interaction of Molecular Oxygen on LaO Terminated
Surfaces of La2NiO4
Taner Akbay,*a Aleksandar Staykov,b John Druce,b Helena Téllez,b
Tatsumi Ishiharaabd and John A. Kilner*bc
a.
Advanced Research Centre for Electric Energy Storage,
Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
b.
International Institute for Carbon Neutral Energy Research (WPI-I2CNER),
Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
c.
Department of Materials,
Imperial College London, South Kensington, London SW7 2BP, United Kingdom
d.
Department of Applied Chemistry,
Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
*
Corresponding authors:
TA (Email: [email protected], Phone: +81 (0)92 802 2870)
JAK (Email: [email protected], Phone: +44 (0)207 594 6745)
1
ABSTRACT
Rare-earth metal oxides with perovskite-type crystal structures are under consideration for use as
air electrode materials for intermediate to high temperature electrochemical device applications.
The surface chemistry of these materials plays a critical role in determining the kinetics of
oxygen reduction and exchange reactions. Among various perovskite-structured oxides, certain
members of the Ruddlesden-Popper series, e.g. La2NiO4, have been identified as significantly
active for surface oxygen interaction. However, the challenge remains to be the identification of
the structure and composition of active surfaces, as well as the influence of these factors on the
mechanisms of surface exchange reactions. In this contribution, the changes in electronic
structure and the energetics of oxygen interaction on surfaces of La2NiO4 are analysed using first
principle calculations in the Density Functional Theory (DFT) formalism. As for the surface
chemistry, the LaO termination rather than the NiO2 is presumed due to recent experimental
evidence for the surfaces of various perovskite structured oxides after heat treatment in oxidizing
environments being transition metal free. Our findings substantiate that the LaO terminated
surface can indeed participate in the formation of the surface superoxo species. Detailed charge
transfer analyses revealed that it is possible for such a surface to be catalytically active owing to
the enhanced electronic configurations on neighbouring La sites to surface species. In addition,
positively charged oxygen vacancies, relative to the crystal lattice, can act as active sites and
catalyse the O-O bond cleavage.
Keywords
Perovskite surfaces, Ruddlesden-Popper phases, oxygen reduction, electro-catalysis, Density
Functional Theory.
1. INTRODUCTION
Mixed ionic electronic conducting (MIEC) materials are attracting considerable attention to
overcome performance limitations of low to intermediate temperature electrochemical devices
for catalytic gas conversion, oxygen separation, and power generation applications. All these
applications require selectively active components made up of materials with substantial oxygen
ion and electron transport properties accompanied by high interfacial activity for electrocatalytic oxygen reduction and incorporation. Such properties are often found in complex oxides
possessing perovskite type crystal structures. The Ruddlesden-Popper (RP) series of compounds,
represented by the general formula of An+1BnO3n+1 (n=1, 2, 3 ...), form a large group of materials
with intriguing properties due primarily to distinct crystallographic layers.1 Here, A and B
2
cations represent the lanthanides (or rare earth metals) and the first row transition metals
respectively. The crystal structure of these series are composed of alternating rock-salt (AO) and
n consecutive perovskite (ABO3) layers along the long axis. The first member of one RP series,
La2NiO4 is a promising MIEC cathode material possessing fast oxygen transport properties.2-6
La2NiO4+ has two structural polymorphs, the phase stability of which are dependent on
temperature and oxygen non-stoichiometry (). At temperatures above 700K, it crystallises as a
tetragonal polymorph belonging to I4/mmm space group symmetry, whilst below 700K, the
crystal structure is found to be orthorhombic possessing Bmab space group symmetry.7, 8 The
most important consequence of the layered nature of the La2NiO4 is the ability to accommodate
varying degrees of over-stoichiometry (0 < < 0.2) through oxygen intercalation into the LaO
rock-salt bilayers. Interstitial oxygen ions largely contribute to the conduction of charge carriers
via the rapid diffusion mechanism along the apical positions of NiO6 corner sharing octahedra in
the perovskite layers.9-13
In solid oxide fuel and/or electrolyser cells (SOFCs & SOECs), the most important requirement
for achieving high conversion efficiencies at low to intermediate operating temperatures is
identified as the facile kinetics of electrode reactions. In particular, the oxygen reduction
reaction (ORR) is considered a major rate limiting factor for intermediate temperature
operation.14-16 Despite concerted experimental and theoretical efforts, the knowledge as to the
actual mechanism of the ORR on such electrode material surfaces is relatively scarce. In
addition, the experimental information on the surface structure and composition at operating
temperatures of SOFC and steam electrolysers are somewhat uncertain. Although the claims lack
experimental validation, it is widely assumed that the outer most layer of the perovskite
structured oxide electrodes are generally made up of B-site cations and O anions (i.e. “BO2
terminated”), owing to the anticipated high catalytic activity of the transition metal cation. Only
recently, it has been shown that the surfaces of perovskite and perovskite-related oxides are
dominated by AO termination at SOFC cathode operating conditions.17-21 The discovery
represents a significant challenge for computational science which has to elucidate the chemical
mechanisms that take place on that particular surface termination.22, 23 In addition, theoretical
investigations suggest that relative stabilities of surfaces of perovskite related oxides are
energetically equivalent for both terminations.24, 25
In an earlier investigation employing first principle techniques, the interaction of an oxygen
molecule with La2NiO4 surfaces was reported by Zhou et al.26 In their work, the mechanisms of
3
oxygen adsorption were elucidated on model surfaces cleaved along <100> direction. Hence, all
defect free and defective surfaces in that study, included Ni2+ cations which further activated the
chemisorbed O2 molecule to superoxo or peroxo state by readily donating electrons. In addition,
Zhou et al. elucidated that the positively charged oxygen vacancies would significantly increase
the O-O bond distance which might lead to oxygen dissociation.
Recently, we used the DFT approach to investigate the oxygen reduction reaction on the SrOterminated surfaces of SrTiO3 and SrTi0.75Fe0.25O3-δ.27 Experimental observations made by Low
Energy Ion Scattering (LEIS) spectroscopy measurements, which can determine the elemental
composition of the outermost layer28, identified the stable surfaces of these materials at elevated
temperatures as predominantly SrO terminated. Our analyses have shown that the oxygen
molecule is not activated by the defect free surface and the reaction is highly endothermic (3.4
eV). This is attributed to the rather inert character of the A-site (Sr) cation when participating in
the electron exchange with adsorbed species. The low energy defective surfaces characterised by
oxygen vacancies, on the other hand, are found to be active for producing superoxo species as a
result of interaction between the dz2 atomic orbitals of the subsurface Ti cation (below the
oxygen vacancy) and the π* molecular orbital of the O2 molecule.
The objective of the present study is to shed light to our understanding on the reactivity of
La2NiO4 surfaces with oxygen by using atomistic simulations. DFT calculations are carried out
for diatomic oxygen over the LaO terminated (001) surface. The effect of surface defects,
characterised by oxygen vacancies, on the energetics of the oxygen adsorption and dissociation
reaction on this particular termination is also elucidated. The catalytic activity of both defect free
and defective surfaces are studied on a model structure based on the tetragonal polymorph of
La2NiO4. Our purpose is to compare the oxygen dissociation reaction mechanisms on the SrO
terminated surface of SrTiO3 and the LaO terminated surface of La2NiO4. Although both
surfaces are terminated by their respective A-site cations, with both Sr and La being large and
inert cations, there is an important difference in their electronic structure. Sr has the electronic
configuration of [Kr]5s2 while La has the electronic configuration of [Xe]5d16s2. The d-electron
occupancy in the valence shell of La allows for polarisation of the shell, including polarisation or
partial hybridisation of the 6s orbitals and easier interaction with the π-shape frontier orbitals of
the oxygen molecule. This interesting difference, and the reported data in the literature for the
superior performance of La-based perovskites as oxygen electrode materials for SOFC and
SOEC, motivated us to elucidate the differences in the reactivity of those surfaces.
4
2. COMPUTATIONAL METHOD
Electron configurations and energetics of a series of model structures were calculated by using
the spin polarised plane wave DFT method with the generalised gradient approximation (GGA)
parametrised by Perdew-Berke-Ernzerhof as implemented in the Vienna ab inito Simulation
Package (VASP).29-32 Projector augmented wave (PAW) pseudopotentials with a kinetic energy
cutoff of 520 eV were employed.33, 34 In order to accurately describe the strong correlation
effects for the transition metal (Ni), a rotationally invariant GGA+U approach (as introduced by
Dudarev) was used with an effective Hubbard parameter of 6 eV.35, 36 A 3 x 3 x 3 Gamma
centered k-point mesh with Blöchl corrections was used to sample the Brillouin zone when
optimizing the tetragonal unit cell.37 The VESTA software package was utilised for the analysis
and visualisation of the structures and the electron density distributions computed by DFT.38
The La2NiO4 surface is represented by a slab supercell made up of 2 x 2 x 4/3 unit cells, which
was utilised for all the simulations to probe the energetics of the surface reaction. In order to
obtain a symmetric slab with no net dipole moment perpendicular to its surface, 9 alternately
charged planes (3 x (LaO-NiO2-LaO)) are stacked along the <001> axis. A vacuum layer of 12
Å is placed over the slab to avoid interaction between neighbouring supercells. A 3 x 3 x 1 kpoint mesh was found to be convergent for accurate sampling of the supercell. The coordinates
of ions in the bottom two planes were constrained to the optimised bulk positions while the rest
are allowed to relax fully. The adsorption of oxygen gas was examined by placing one oxygen
molecule per 2 x 2 unit cell surface area, roughly corresponding to a surface coverage of 25%.
Two distinct 2 x 2 surface models were constructed: a defect free slab with stoichiometric
number of ions and a defective slab with one oxygen ion removed from each of the top and the
bottom layers of the slab. The convergence criteria for electronic self-consistency and ionic
relaxation cycles were set as 10-4 eV/atom and 10-2 eV/Å respectively. Ferromagnetic (FM)
ordering was chosen as the initial magnetic moment configuration for all atoms in the model
structure. Bader population analysis was performed on the charge density grid to calculate the
effective atomic charges.39
The energy barriers for oxygen dissociation were calculated by employing the climbing image
nudged elastic band (CI-NEB) method.40-45 In order to map the potential energy barriers, six
linearly interpolated images were generated as intermediate structures between the two
energetically stable start and end configurations, corresponding to the adsorbed and dissociated
5
states of oxygen on the model surface respectively. Only ions were allowed to relax, with the
lattice vectors kept fixed during the NEB calculations.
3. RESULTS & DISCUSSION
Figure 1a depicts the optimised geometry of the La2NiO4 crystal. The lattice parameters of the
fully relaxed unit cell of the bulk La2NiO4 crystal structure were calculated as a = b = 3.904 Å, c
= 12.539 Å, and = = = 90○, which are within 1% error from reported experimental data.2
No noticeable change is observed in the fractional coordinates of the ionic positions in the
relaxed structure. The tetragonal distortion (Jahn-Teller distortion) measured by the ratio of bond
lengths of Ni-Oa (2.238 Å) to Ni-Oe (1.952 Å) is found as 1.15 (Oa and Oe are used to denote the
apical and the equatorial oxygens in the NiO6 octahedron respectively). This causes a change in
the surface electronic structure with significant charge redistribution between Ni, La and Oa
atoms. The fully relaxed slabs for defect free and defective surfaces, shown in Figures 1b and 1c,
followed a well-established trend for cleaved surfaces with contraction in the surface layer and
expansion in the sub-surface layer towards vacuum.
Figure 1. (a) La2NiO4 crystal lattice, (b) LaO terminated slab models of defect-free 2 x 2 (001)
surface, and (c) defective 2 x 2 (001) surface. Oxygen vacancies are placed on the top and
bottom layers of the defective slab in body diagonal fashion to maximise the distance between
them. La is depicted green, Ni is depicted grey, O is denoted red.
6
As described by Tasker, the charged surfaces of ionic solids with a net dipole moment have an
infinitely large surface energy.46 Therefore, these surfaces cannot exist and their natural
occurrence require major reconstruction involving foreign species and/or defect restructuring to
stabilise them. Tasker categorised charged surfaces into three distinct types according to the
stacking sequences and the total electrostatic potentials of subsurface layers. When the sum of
electrostatic potentials over anionic and cationic sublattices on individual planes of an ionic
crystal is zero, the surface is labelled as type 1 and has no net dipole moment perpendicular to it.
If the crystal is made up of charged subsurface layers grouped and stacked in symmetrical
configurations, the Madelung sums for the potential of any lattice site becomes finite with a
diminishing overall dipole moment perpendicular to its surface. Such surfaces are referred to as
type 2 surfaces according to Tasker’s classification. Type 3 surfaces, however, have finite dipole
moment values perpendicular to them due to the charged subsurface layers stacked in nonsymmetrical fashion.
In this work, both defect free and defective LaO terminated surfaces are obtained by cleaving the
bulk crystal along the <001> direction. Since the individual subsurface layers perpendicular to
the long axis are not neutral, the slab model is created as a type 2 Tasker surface with 9
alternating layers composed of 3 LaO double layers and 3 NiO2 layers while maintaining the
symmetry to eliminate the net dipole moment perpendicular to its surface. Even though the LaO
terminated surfaces can also be stabilised by La vacancies, as described for the case of LaAlO3
47
, here we have chosen the cation defect free termination as a simplified model of the real
surface.
3.1 Oxygen adsorption on LaO terminated defect-free (001) surface:
Gaseous oxygen is one of the few molecules characterised by a high-spin ground state and lowspin excited state. The reason for this lies in the electronic structure of di-oxygen, which is
characterised by two degenerate antibonding  molecular orbitals formed by out of phase
overlap between the 2p atomic orbitals on the oxygen atoms. The  molecular orbitals are
spatially orthogonal with zero overlap which leads to the high stability of the triplet spin state of
O2 with a single electron occupying each  molecular orbital. The O-O bond distance for an
isolated O2 molecule is 1.21 Å. The triplet O2 interacts with the surface magnetic moment
implying that the adsorption and dissociation reactions would involve various spin state
7
transformations.48, 49 In order to participate in a chemical reaction, the symmetry of the 
molecular orbitals should be broken either by thermal or irradiative oxidation to the less stable
singlet excited state, or through electron transfer to one or both  molecular orbitals. A single
electron transferred to the O2 molecule leads to the highly reactive superoxo state which is an
anion radical (O2-.), and two electrons transferred to the O2 molecule lead to the mildly oxidant
peroxo state (O22-). As both the  molecular orbitals of O2 are a result of out-of-phase overlap of
the 2p atomic orbitals on the oxygen atoms, any additional electron occupation of those orbitals
leads to repulsion between the oxygen atoms and would be a precursor state for oxygen
dissociation. In the superoxo state, the O-O bond length is 1.3~1.4 Å, whilst in the peroxo state,
the O-O bond length is 1.4~1.6 Å. The relative orientation of the O2 molecule with respect to the
surface also becomes an important factor to determine the energetically most favorable path to
transfer charge from the surface to the  orbitals.
A number of distinct geometric orientations for the O2 molecule are considered in this study. For
all cases, the O2 molecule is placed 2 Å above the surface with an initial guess of triplet spin
state. The initial magnetic state of the slab, on the other hand, is assumed to be ferromagnetic.
Among all calculated adsorption geometries, two energetically favorable orientations with sideon configurations are determined. The first corresponds to the adsorption of O2 to the slip
position on a La atom and the second one corresponds to adsorption to a La-La bridge as
depicted in Figures 2a and 2b respectively.
8
Figure 2. (a) Top view of the defect free La2NiO4 slab with adsorbed O2 on the slip position, and
(b) on the La-La bridge position. (c) & (d) Electron probability density distribution contours
around the adsorbed oxygen. Colour scale: Blue=0, Red=0.15 Å-3. (e) & (f) Electron density
difference plots for the chemisorbed oxygen. Positive and negative electron density differences
are denoted by blue and yellow respectively. The adsorbed O2 molecule is depicted as purple and
only the top perovskite layer of the slab is shown for clarity.
The total energies of the La2NiO4 slab with O2 adsorbed to a slip position on La and side on to a
La-La bridge position were calculated as -625.94 eV and -625.80 eV respectively. The energies
in both geometries are comparable, but the O2 chemisorbed on the slip position is found to be
slightly more stabilised (~0.14 eV). The O-O bond length for O2 on the slip position is 1.46 Å.
Similarly, the bond length of O2 on the La-La bridge position is 1.33 Å. This suggests that the
adsorbed oxygen at bridge position is in the superoxo activated state, while in the slip position it
is further activated to the peroxo state. The distances between either of the O and the nearest La
atom for each configuration are 2.39 Å and 2.53 Å respectively.
9
The charges within the slab and the electron transfer to the chemisorbed oxygen molecules on
the surface was estimated by applying Bader population analysis (Atoms in Molecules method).
Interestingly, the average charge of La in the lattice is found to be +2, the charge of Ni is +1.3,
the charge of lattice O is -1.3 and the charges of the oxygen atoms in the oxygen molecule are 0.47 and -0.27 for the La-La bridge case, and -0.69 and -0.58 in the slip position. The Bader
charges are lower than the formal charges of La2NiO4 where La, Ni, and O are assumed to have
charges of +3, +2, and -2, respectively. The difference between the formal ionic charges and the
estimated Bader charges shows the significant degree of covalent bonding within the lattice and
resulting from shared electron pairs. It is important to note that not only Ni but also La deviates
from its formal ionic charge. Such an effect was not observed in SrTiO3 where the A-site cation
Sr has Bader charge corresponding to its formal ionic charge +2.27 The ionicity of the Sr
excludes it from the shared electron network of TiO2 and prevents it from mediating possible
charge transfer between Ti and any species adsorbed on the SrO surface. In case of La2NiO4 the
Bader analysis shows that the La is an active part of the shared lattice electron network. The
charge on the oxygen molecule is -0.74 for the La-La bridge and -1.27 for the slip position.
Those results show that for the La-La bridge position the oxygen molecule has an electronic
structure close to superoxo state while in the slip position its electronic structure is closer to
peroxo state. The Bader analysis of the atoms within the lattice suggests the electron density is
donated from the electronic structure of the entire lattice and not from a given species on the
surface. Analysing the charge density plots in Figures 2c and 2d we can conclude that shared
electron density exists between the oxygen atoms from the oxygen molecule and the La atoms
on the surface, suggesting that the O2 adsorption is indeed a stable chemisorption state. Such
stable chemisorption was not found for SrTiO3, which is another manifestation for the improved
catalytic activity of the LaO surface50-53 compared to the SrO surface related to the electronic
properties of La. Analysis of the electron probability density in Figures 2c and 2d suggests an
overlap between the oxygen  molecular orbital and the d-orbitals of the La atoms.
This assumption is further proved by the electron density difference plots in Figures 2e and 2f
where the electron density between the oxygen atoms of the oxygen molecule is reduced and the
electron density of the  molecular orbital is increased. In Figures 2e and 2f positive electron
density is denoted in blue and negative electron density is denoted by yellow. It is important to
note that electron density is donated not only from the surface La atoms but also from the NiO2
(BO2) network in the subsurface layer, which acts as an electronic reservoir for the dissociative
adsorption process, mediated by La atoms in the terminating surface layer.
10
In addition to the above-mentioned minimum energy geometries for the chemisorption of an
oxygen molecule, a variety of possible configurations for the dissociated oxygen atoms were
tested. Among these, the minimum energy configurations were found to involve monoatomic
oxygen on a bridge site between two La atoms as shown in Figures 3a and 3b. Total energies of
the La2NiO4 slab are calculated as -625.84 eV, and -624.24 eV respectively for these
configurations, both of which higher than the energy of the chemisorbed O2 on the surface.
11
Figure 3. (a) – (b) Top views of the defect free La2NiO4 slab with dissociated oxygen on La-La
bridge positions of the LaO terminated (001) surface. (c) – (d) Electron probability density
distribution contours around the dissociated oxygen. Colour scale: Blue=0, Red=0.15 Å-3. (e) (f) Electron density difference plots for the dissociated oxygen. Positive and negative electron
densities are denoted by blue and yellow respectively. The dissociated oxygen atoms are
depicted as purple.
Figures 3c and 3d show the electron density map of the adsorbed oxygen atoms over the
La2NiO4 LaO terminated surface. Both density maps suggest that there is binding interaction
between surface La atoms and the adsorbed oxygen atom characterised by electronic density
shared between the La and O nuclei. Such interaction was not found in case of SrTiO3. The
adsorbed oxygen on the SrTiO3 was highly reactive, with a tendency to recombine into
molecular oxygen, and was the main reason why the defect free SrTiO3 could not participate in
the oxygen activation reaction. The reason was that Sr atoms could not stabilise the atomic
oxygen by electron transfer. However, La on the surface could donate electron density to the
atomic oxygen and thus stabilise dissociated oxygen atoms on the surface. Bader analysis
indicates the charge of the oxygen atom on the surface is -1 electron. The assumption that La on
the surface plays a stabilising role is further supported by the electron density difference plots
depicted in Figures 3e and 3f. In both figures it can be seen that negative electron density
(denoted with yellow colour) at La atoms points towards the adsorbed oxygen atoms and
positive electron density (denoted with blue colour) is localised at the adsorbed oxygen atoms.
The electron density difference maps clearly demonstrate the charge transfer from surface La
atoms to adsorbed oxygen atoms, stabilising the surface oxygen species.
12
Figure 4. Results of the transition state analysis between the chemisorbed O2 on the slip position
and the dissociated oxygen on the La-La bridge position on defect free surface. The reference
energy configuration is the approach of molecular oxygen to the surface. The activation energy
for the forward reaction is 1.35 eV while the backward reaction requires 0.25 eV.
In order to probe the energetics of transition states between the chemisorbed O2 at the slip
position and the dissociated oxygen, the CI-NEB analyses were performed. The energy diagram
is shown in Figure 4. The reference energy is chosen as the energy of the slab with the oxygen
molecule 3.3 Å above the surface, at which distance the interaction between the oxygen
molecule and the surface can be neglected. As the oxygen molecule approaches the surface, the
energy decreases by 0.73 eV, showing that the surface has strong affinity towards oxygen
chemisorption. The oxygen molecule is chemisorbed on a La-slip site. This energy calculation
verifies our electronic density analysis in Figure 2c, showing that a binding interaction between
the surface and the oxygen molecule occurs, and the binding strength corresponds to 0.73 eV.
The transition state for the oxygen dissociation has an energy barrier of 1.35 eV which is
comparable to the barrier estimated from a similar analysis on the surface of SrTiO3 27, and with
experimental results reported in the literature.54, 55 The dissociated oxygen atoms are stabilised
compared to the transition state energy by 0.25 eV, which is the main difference from the
dissociation mechanism on the SrTiO3 surface where such stabilisation was not observed. The
lowest energy is estimated for the chemisorbed oxygen molecule on the LaO surface and while
13
the LaO terminated La2NiO4 surface can dissociate oxygen, the lower activation barrier for the
reverse reaction shows that the surface would tend to favour oxygen evolution.
Figure 5. Results of the transition state analysis between the chemisorbed O2 on the La-La
bridge position and the dissociated oxygen on the La-La bridge position on defect free surface.
The reference energy configuration is the approach of molecular oxygen to the surface. The
activation energy for the forward reaction is 1.095 eV while the backward reaction requires
0.535 eV.
Another possible reaction path is shown in Figure 5 with the oxygen molecule being
chemisorbed on the La-La bridge site. Similar to the case represented in Figure 4, the reference
energy was chosen to be the oxygen molecule at 3.3 Å above the surface. As a result of the
chemisorption, the energy was stabilised by 0.59 eV which is slightly lower than that of the
chemisorption at a La slip position (0.73 eV). The oxygen dissociation from this configuration
requires an energy barrier of 1.09 eV. The activation barrier for the reverse reaction is 0.53 eV.
It is worth noting that the energy of the end product is lower compared to the previous case
shown in Figure 4, -0.03 eV and 0.37 eV, respectively. This difference can be attributed to the
separation between the two dissociated oxygen atoms on the surface. In the case of the La slip
position (Figure 4) the two dissociated oxygen atoms are stabilised by electron transfer from
three surface La atoms while in the case of the La-La bridge site (Figure 5), the two dissociated
oxygen atoms are stabilised by electron transfer from two surface La atoms. This result shows
14
that the dissociated oxygen atoms on the La2NiO4 surface tend to occupy distant positions in
order to withdraw maximum electron density from the surface.
3.2 Oxygen adsorption on LaO terminated defective (001) surface:
Moving on from the ideal surfaces discussed above, we investigated O2 adsorption on defective
surface by using a La2NiO4 slab with an oxygen vacancy on its LaO terminated (001) surface.
Similar to the previously presented cases for the defect free slab, a number of different initial
geometric orientations for the O2 molecule were considered (see the Supporting Information).
All possible starting configurations converged to the final end-on O2 configuration as depicted in
Figure 6a. One of the oxygen atoms of the molecule occupy the vacant site, while the second one
points towards the La-La bridge. The oxygen-oxygen bond within the oxygen molecule is 1.5 Å
which suggests that surface-molecule electron transfer has occurred and the oxygen molecule
has been activated to the peroxo state. The bond distance between the Ni atom beneath the
oxygen vacancy and the oxygen atom from the oxygen molecule is 2.2 Å, which is close to the
nominal Ni-O distance at the surface (2.1 Å). Figure 6c shows a cut plane through the total
electron density displaying the O-O bond and the electronic interaction between the oxygen
molecule, the Ni beneath the oxygen vacancy, and the neighbouring La atoms.
15
Figure 6. (a) Top view of the defective La2NiO4 slab with O2 adsorbed onto the oxygen
vacancy, (b) incorporated oxygen into the vacancy and dissociated oxygen atoms on the La-La
bridge position. (c) – (d) Electron probability density distribution contours around the
incorporated and adsorbed oxygen. Colour scale: Blue=0, Red=0.15 Å-3. (e) - (f) Electron
density difference plots for the chemisorbed oxygen. Positive and negative electron densities are
denoted by blue and yellow respectively.
The electron density forming the O-O bond within the oxygen molecule can be clearly seen,
verifying the oxygen molecule has not yet dissociated. Additional weak interaction can be seen
between the Ni and the O atoms which is similar to the Ti-O interaction estimated previously for
iron doped SrTiO3.27 A major difference between the La2NiO4 and SrTiO3 is that the surface La
atoms interact with the oxygen molecule, supporting the surface molecule chemisorption. In the
case of SrTiO3 such interaction was not observed. We attribute this additional interaction to the
electronic structure of La where the additional d-electron in the valence shell leads to
polarisation of the valence shell s-orbitals. The Bader population analysis shows that -1.44
electrons are transferred from the surface to the activated molecule (-0.75 electrons for the O in
16
the vacancy and -0.69 for the O on the La-La bridge). This verifies our assumption that the
oxygen molecule is in activated peroxo state. The surface to molecule charge transfer is greater
for the defective surface compared to the defect free surface, showing that the surface vacancies
can play a significant role in the oxygen activation.
Figure 6e shows the electron density difference map for the oxygen molecule chemisorbed into
the vacancy. Blue colour denotes positive electron density difference (i.e. excess electrons
relative to the base case), whilst yellow denotes negative electron density difference (i.e.
depletion of the electron concentration). The results show the Ni beneath the vacant site loses
electrons, which become localised around each oxygen atom. The results resemble the electron
density distribution of an oxygen molecule chemisorbed in oxygen vacancy on the SrTiO3
surface.27 Analysis of the electron density differences show that electron density is transferred
between the dz2 orbital of Ni (or Ti in SrTiO3) and the π* antibonding orbital of the oxygen
molecule. An important difference in the La2NiO4 case is that the surface La atoms also take part
in the electron density transfer to the oxygen molecule. Such charge transfer was not observed
via the surface Sr atoms in the case of SrTiO3.27
Figure 6b shows the geometry of the oxygen molecule dissociated on the surface with one
oxygen atom incorporated in the perovskite lattice and one oxygen atom on the La-La bridge
position. The charge at each oxygen atom is -1.3 which is similar to the charge of the oxygen
atoms in the lattice. Figure 6d shows a plane section through the total electron density, passing
through the chemisorbed oxygen atom and the oxygen atom on the La-La bridge site. The
electron density in Figure 6d verifies the existence of strong La-O interaction characterised by
charge transfer in the inter-nuclear region, which suggests a strong binding interaction. The
electron density differences plot is shown in Figure 6f. Again, the blue colour denotes positive
electron density difference whilst yellow denotes negative electron density difference. The
density difference plots show that the oxygen incorporated into the perovskite lattice receives
electron density drawn from the entire NiO2 sub-lattice. The oxygen atom chemisorbed on the
surface receives electron density from the bridge La atoms at the surface. This leads to a
significant stabilisation of the dissociated oxygen on the LaO terminated surface, which was not
observed for oxygen absorbed on the comparable SrO terminated SrTiO3 surface.27
17
Figure 7. Results of the transition state analysis between the chemisorbed O2 on the oxygen
vacancy and the incorporated/dissociated oxygen on the La-La bridge positions on defective
surface. The dissociation of oxygen proceeds with a barrier of 0.28 eV.
In order to identify the transition state between the chemisorbed and dissociated oxygen
molecule on the defective surface, and the associated activation barrier, a set of CI-NEB
analyses were performed between the geometries in Figure 6a and 6b. The results for the
transition state search are shown in Figure 7. The activation barrier for the dissociation of the
chemisorbed oxygen was found to be 0.28 eV. The dissociation reaction is exothermic and the
energy of the product is 0.93 eV lower than the reactant. The reaction barrier for the reverse
reaction is 1.21 eV. The reaction can proceed exothermically because of the electron transfer
from the La atoms to the surface chemisorbed oxygen. The activation barrier for oxygen
dissociation on the defective La2NiO4 surface is lower compared to the barrier on the defect free
La2NiO4 surface, 0.28 eV and 1.09 eV, respectively. Also the activation barrier for oxygen
dissociation on the defective La2NiO4 surface is lower compared to the barrier on the defective
SrTiO3 surface, 0.28 and 0.5 eV, respectively.
4. CONCLUSION
Following recent experimental reports that the external surfaces of perovskite materials to be AO
terminated when exposed to the operating conditions encountered in their application in devices
such as SOFC cathodes, we studied the energetics of oxygen adsorption and dissociation
reactions on the LaO terminated surfaces of La2NiO4. We have demonstrated that the defect free
18
(001) surface, with this particular termination, is actually energetically favorable for oxygen
adsorption and dissociation. The activation energies for the oxygen dissociation are calculated as
about 1 eV by using DFT+U and CI-NEB calculations. In contrast to common assumption, the
electronic structure of the LaO terminated surface can promote charge transfer from the surface
La atoms to the adsorbed oxygen. This finding, however, cannot be generalised for all cations
that occupy the A site in perovskite phases. In our recent study, for example, SrO terminated
defect free (001) surface of SrTiO3 was found to be energetically not favorable for oxygen
adsorption and dissociation. This strongly suggests that the electronic structure of the A-site
cations plays a very important role in promoting charge transfer to surface adsorbates. In the
case of Sr, the valence shell is built by 5s electrons. That atomic orbital is easily ionized leading
to the Sr2+. The closed shell Sr2+ does not interact with chemisorbed oxygen molecules and
therefore does not provide electron density that would facilitate the oxygen dissociation reaction.
In the case of La, however, the valence shell is built by the 6s and one 5d electrons. The
additional d-electron in the valence shell leads to the polarisation of the 6s valence orbitals.
Thus, La can facilitate the oxygen chemisorption and destabilise the oxygen molecule by surface
to molecule charge transfer. We believe that this comparison sheds light to our understanding on
why the superior catalytic activity in oxygen reduction reactions is observed in La perovskite
oxides. These findings have far reaching implications in the fundamental understanding of the
interaction of oxygen with complex metal oxide surfaces.
ACKNOWLEDGMENTS
TA is grateful for the support of the Advanced Research Centre for Electric Energy Storage
sponsored by Kyushu University. The support of the International Institute for Carbon Neutral
Energy Research (WPI-I2CNER) sponsored by the Ministry of Education, Culture, Sports,
Science and Technology (MEXT) of the Japanese government is also acknowledged. Part of this
study was financially supported by the Grant-in-Aid for Specially Promoted Research (No.
16H06293) from MEXT, Japan.
REFERENCES
1. S. N. Ruddlesden, P. Popper, “New compounds of the K2NiF4 type”, Acta Crystallogr.,
1957, 10, 538-529.
2. S. J. Skinner, J. A. Kilner, “Oxygen diffusion and surface exchange in La2-xSrxNiO4+”,
Solid State Ionics, 2000, 135, 709-712.
19
3. R. Sayers, R. A. De Souza, J. A. Kilner, S. J. Skinner, “Low temperature diffusion and
oxygen stoichiometry in lanthanum nickelate”, Solid State Ionics, 2010, 181, 386-391.
4. M. Burriel, G. Garcia, J. Santiso, J. A. Kilner, R. J. Chater, S.J. Skinner, “Anisotropic
oxygen diffusion properties in epitaxial thin films of La2NiO4+”, J. Mater. Chem., 2008,
18, 416.
5. J. M. Bassat, P. Odier, A. Villesuzanne, C. Marin, M. Pouchard, “Anisotropic ionic
transport properties in La2NiO4+ single crystals”, Solid State Ionics, 2004, 167, 341-347.
6. E. Boehm, J.-M. Bassat, P. Dordor, F. Mauvy, J.-C. Grenier, Ph. Stevens, “Oxygen
diffusion and transport properties in non-stoichiometric Ln2-xNiO4+ oxides”, Solid State
Ionics, 2005, 176, 2717-2725.
7. J. Rodrigez-Carvajal, J. L. Martinez, J. Pannetier, R. Saez-Puche, “Anomalous structural
phase transition in stoichiometric La2NiO4”, Phys. Rev. B, 1988, 38, 7148.
8. S. J. Skinner, “Characterization of La2NiO4+ using in-situ high temperature neutron
powder diffraction”, Solid State Sci., 2003, 5, 419-426.
9. L. Minervini, R. W. Grimes, J. A. Kilner, K. E. Sickafus, “Oxygen migration in
La2NiO4+”, J. Mat. Chem., 2000, 10, 2349.
10. A. R. Cleave, J. A. Kilner, S. J. Skinner, S. T. Murphy, R. W. Grimes, “Atomistic
computer simulation of oxygen ion conduction mechanism in La2NiO4”, Solid State
Ionics, 2008, 179, 823-826.
11. J. H. Lee, G. Luo, I. C. Tung, S. H. Chang, Z. Luo, M. Malshe, M. Gadre, A.
Bhattacharya, S. M. Nakhmanson, J. A. Eastman, H. Hong, J. Jellinek, D. Morgan, D. D.
Fong, “Dynamic layer rearrangement during growth of layered oxide films by molecular
beam epitaxy”, Nature Materials, 2014, 13, 879-883.
12. D. E. E. Deacon-Smith, D. O. Scanlon, R. A. Catlow, A. A. Sokol, S. M. Woodley,
“Interlayer cation exchange stabilizes polar perovskite surfaces”, Adv. Mater., 2014, 26,
7252-7256, DOI: 10.1002/adma.201401858.
13. M. S. D. Read, M. S. Islam, G. W. Watson, F. E. Hancock, “Surface structures and defect
properties of pure and doped La2NiO4”, J. Mater. Chem., 2011, 11, 2597-2602.
14. R. Barfod, A. Hagen, S. Ramousse, P.V. Hendriksen, M. Mogensen, “Break down of
losses in thin electrolyte SOFCs”, Fuel Cells, 2006, 6(2), 141-145, DOI:
10.1002/fuce.200500113.
20
15. J. T. S. Irvine, D. Neagu, M. C. Verbraeken, C. Chatzichristodoulou, C. Graves, M. B.
Mogensen, “Evolution of the electrochemical interface in high-temperature fuel cells and
electrolysers”, Nature Energy, 2016, 1, 1-13.
16. S.B. Adler, “Factors governing oxygen reduction in solid oxide fuel cell cathodes”,
Chem. Rev., 2004, 104, 4791-4843.
17. J. Druce, H. Téllez, M. Burriel, M. D. Sharp, L. J. Fawcett, S. N. Cook, D. S. McPhail, T.
Ishihara, H. H. Brongersmac, J. A. Kilner, “Surface termination and subsurface
restructuring of perovskite-based solid oxide electrode materials”, Energy Environ. Sci.,
2014, 7, 3593-3599.
18. J. Druce, T. Ishihara, J. A. Kilner, “Surface composition of perovskite-type materials
studied by Low Energy Ion Scattering (LEIS)”, Solid State Ionics, 2014, 262, 893-896.
19. H. Téllez, J. Druce, Y.-W. Ju, J. A. Kilner, T. Ishihara, “Surface chemistry evolution in
LnBaCo2O5+δ double perovskites for oxygen electrodes”, Int. J. Hydrog. En., 2014, 39,
20856-20863.
20. J. Druce, H. Téllez, N. Simrik, T. Ishihara, J. A. Kilner, “Surface composition of solid
oxide fuel cell electrode structures by laterally resolved low energy ion scattering
(LEIS)”, Int. J. Hydrogen Energy, 2014, 39, 20850-20855, DOI:
10.1016/j.ijhydene.2014.07.005
21. M. Burriel, S. Wilkins, J. P. Hill, M. A. Munoz-Marquez, H. H. Brongersma, J. A.
Kilner, M. P. Ryan, S. J.Skinner, “Absence of Ni on the outer surface of Sr doped
La2NiO4 single crystals”, Energy Environ. Sci., 2014, 7, 311-316.
22. D. Lee, Y.-L. Lee, A. Grimaud, W. T. Hong, M. D. Biegalski, D. Morgan Y. Shao-Horn,
“Strontium influence on the oxygen electrocatalysis of La2-x Srx NiO4+/-δ (0.0 < x < 1.0)
thin films”, J. Mater. Chem. A, 2014, 2, 6480-6487.
23. X. Ma, J. S. A. Carneiro, X.-K. Gu, H. Qin, H. Xin, K. Sun, E. Nikolla, “Engineering
complex, layered metal oxides: High-performance nickelate oxide nanostructures for
oxygen exchange and reduction”, ACS Catal., 2015, 5, 4013-4019.
24. S. Woo, H. Jeong, S. A Lee, H. Seo, M. Lacotte, A. David, H. Y. Kim, W. Prellier, Y.
Kim, W. S. Choi, “Surface properties of atomically flat polycrystalline SrTiO3”, Sci.
Rep., 2015, 5, 8822.
25. M. M. Kuklja, E. A. Kotomin, R. Merkle, Yu. A. Mastrikov, J. Maier, “Combined
theoretical and experimental analysis processes determining cathode performance in
solid oxide fuel cells”, Phys. Chem. Chem. Phys., 2013, 15, 5443-5471.
21
26. J. Zhou, G. Chen, K. Wu, Y. Cheng, “Interaction of La2NiO4 (100) surface with oxygen
molecule: A first principles study”, J. Phys. Chem. C, 2013, 117, 12991-12999.
27. A. Staykov, H. Téllez, T. Akbay, J. Druce, T. Ishihara, J. A. Kilner, “Oxygen activation
and dissociation on AO terminated perovskite surfaces”, Chem. Mater., 2015, 27, 82738281.
28. H.H. Brongersma, M. Draxler, M. de Ridder, P. Bauer, “Surface composition analysis by
low-energy ion scattering”, Surface Science Reports, 2007, 62(3), 63-109.
29. G. Kresse, J. Hafner, “Ab initio molecular dynamics for liquid metals”, Phys. Rev. B,
1993, 47, 558.
30. G. Kresse, J. Hafner, “Ab initio molecular-dynamics simulation of the liquid-metalamorphous-semiconductor transition in germanium”, Phys. Rev. B, 1994, 49, 14251.
31. G. Kresse, J. Furthmüller, “Efficiency of ab-initio total energy calculations for metals
and semiconductors using a plane-wave basis set”, Comput. Mat. Sci., 1996, 6, 15.
32. G. Kresse, J. Furthmüller, “Efficient iterative schemes for ab initio total-energy
calculations using a plane-wave basis set”, Phys. Rev. B, 1996, 54, 11169-11186.
33. J. P. Perdew, K. Burke, M. Ernzerhof, “Generalized gradient approximation made
simple”, Phys. Rev. Lett., 1996, 77, 3865-3868.
34. J. P. Perdew, K. Burke, M. Ernzerhof, “Erratum: Generalized gradient approximation
made simple”, Phys. Rev. Lett., 1997, 78, 1396.
35. S. L. Dudarev, G. A. Botton, S. Y. Savrasov, C. J. Humphreys, A. P. Sutton, “Electronenergy-loss spectra and the structural stability of nickel oxide: An LSDA+U study”,
Phys. Rev. B, 1998, 57, 1505-1509.
36. G. Hautier, S. P. Ong, A. Jain, C. J. Moore, G. Ceder, “Accuracy of density functional
theory in predicting formation energies of ternary oxides from binary oxides and its
implication on phase stability”, Phys. Rev. B, 2012, 85, 155208.
37. P. E. Blöchl, “Projector augmented-wave method”, Phys. Rev. B, 1994, 50, 17953-17979.
38. K. Momma, F. Izumi, “VESTA 3 for three-dimensional visualization of crystal,
volumetric, and morphology data”, J. Appl. Crystallogr., 2011, 44, 1272-1276.
39. W. Tang, E. Sanville, G. Henkelman “A grid-based Bader analysis algorithm without
lattice bias”, J. Phys.: Condens. Matter, 2009, 21, 084204.
40. H. Jónsson, G. Mills, K. W. Jacobsen, “Nudged elastic band method for finding
minimum energy paths of transitions, in classical and quantum dynamics in condensed
22
phase simulations”, Ed. B. J. Berne, G. Ciccotti and D. F. Coker, World Scientific, 1998,
p. 385.
41. G. Henkelman, B. P. Uberuaga, H. Jónsson, “A climbing image nudged elastic band
method for finding saddle points and minimum energy paths”, J. Chem. Phys., 2000, 113,
9901.
42. G. Henkelman, H. Jónsson, “Improved tangent estimate in the nudged elastic band
method for finding minimum energy paths and saddle points”, J. Chem. Phys., 2000, 113,
9978.
43. D. Sheppard, R. Terrell, G. Henkelman, “Optimization methods for finding minimum
energy paths”, J. Chem. Phys. 2008, 128, 134106.
44. D. Sheppard, G. Henkelman, “Paths to which the nudged elastic band converges”, J.
Comp. Chem., 2011, 32, 1769-1771.
45. D. Sheppard, P. Xiao, W. Chemelewski, D. D. Johnson, G. Henkelman, “A generalized
solid-state nudged elastic band method”, J. Chem. Phys., 2012, 136, 074103.
46. P. W. Tasker, “The stability of ionic crystal surfaces”, J. Phys. C: Solid State Phys.,
1979, 12, 4977.
47. C. H. Lanier, J. M. Rondinelli, B. Deng, R. Kilaas, K. R. Poeppelmeier, L. D. Marks,
“Surface reconstruction with a fractional hole: (√5 x (√5)R26.6° LaAlO3 (001)”, Physical
Review Letters, 2007, 98, 086102-1-4.
48. G. Y. Guo, W. M. Temmerman, “Electronic structure and magnetism in La2NiO4”, J.
Phys. C. Solid State Physics, 1988, 21, L803.
49. G. Y. Guo, W. M. Temmerman, “Electronic and magnetic properties of La2NiO4: The
importance of La-O planes”, Phys. Rev. B. Condens Matter, 1989, 40, 285-288.
50. C.-H. Lin, K.D. Campbell, J.-X. Wang, J.H. Lunsford, “Oxidative dimerisation of
methane over lanthanum oxide”, Journal of Physical Chemistry, 1986, 90(4), 534-537.
51. A.V. Vishnyakov, I.A. Korshunova, V.E. Kochurikhin, L.S. Sal’nikova, “Catalytic
activity of rare earth oxides in flameless methane combustion”, Kinetics and Catalysis,
2010, 51(2), 273-278.
52. P. Huang, Y. Zhao, J. Zhang, Y. Zhu, Y. Sun, “Exploiting shape effects of La2O3
nanocatalysts for oxidative coupling of methane reaction”, Nanoscale, 2013, 5, 1084410848, DOI: 10.1039/C3NR03617K.
23
53. M.S. Palmer, M. Neurock, M.M. Olken, “Periodic density functional theory study of the
dissociative adsorption of molecular oxygen over La2O3”, J. Phys. Chem. B, 2002, 106,
6543-6547.
54. M. J. Escudero, A. Aguadero, J. A. Alonso, L. Daza, “A kinetic study of oxygen
reduction reaction on La2NiO4 cathodes by means of impedance spectroscopy”, Journal
of Electroanalytical Chemistry, 2007, 611(1–2), 107-116.
55. T. Ogier, F. Mauvy, J. M. Bassat, J. Laurencin, J. Mougin, J. C. Grenier,
“Overstoichiometric oxides Ln2NiO4+δ (Ln = La, Pr or Nd) as oxygen anodic electrodes
for solid oxide electrolysis application”, Int. J. of Hydrogen Energy, 2015, 40(46),
15885-15892.
24